1. Field of the Invention
The present invention relates to a technology for improving tear resistance at a shoulder of a pneumatic radial tire having a thin groove continuous in a tire circumferential direction at a buttress.
2. Description of the Related Art
FIGS. 5A and 5B are schematics of an example of a conventional pneumatic radial tire. As shown in FIG. 5A, in the conventional pneumatic radial tire, particularly in a pneumatic radial tire 100 for heavy loads that is mounted to a truck or a bus, a plurality of circumferential direction grooves 102 that are continuous in a tire circumferential direction are formed in a tread 101. Ribs (blocks) 103 are formed by being demarcated at least by the circumferential direction grooves 102. In the conventional pneumatic radial tire 100, a temporary rib 107 is formed by a thin groove 106 in an end portion 105 of a shoulder 104 of the tread 101. By providing the thin groove 106, it is possible to prevent a ground-contact pressure of a sidewall side end portion (not shown) of the ribs 103 of the shoulder 104 from becoming higher than a ground-contact pressure of the ribs (blocks) at the center of the tire (not shown), and to improve uneven wear resistance of the shoulder 104 of the tread 101.
However, when the truck or the like on which the conventional pneumatic radial tires 100 are installed rides up over a curb or the like at a road surface, the temporary rib 107 formed at the end portion 105 of the shoulder 104 of the tread 101 breaks, i.e., cut-resistance becomes poor. Thus, as shown in FIG. 5B, there has been proposed a pneumatic radial tire 110 that has a thin groove 121 continuous in the tire circumferential direction, at a buttress 120 formed between the tread 101 and a sidewall (not shown). According to the conventional pneumatic radial tire 110, breaking of the rib 103 of the shoulder 104 of the tread 101 can be suppressed.
When rims on which the pneumatic radial tires 110 are mounted are installed at a truck or the like whose distance between axes is long, and the truck turns at an extremely low speed, e.g., when the pneumatic radial tires 110 rotate at the position with respect to the road surface, the following problems arise. FIG. 6A is a schematic for illustrating a relationship between the road surface and the conventional pneumatic radial tire in an unloaded state. FIG. 6B is a schematic for illustrating a relationship between the road surface and the conventional pneumatic radial tire in a loaded state. FIG. 6C is a schematic for illustrating a relationship between the road surface and the conventional pneumatic tire that is in the loaded state and at which lateral force has arisen. When load F is applied to the pneumatic radial tire 110 in the unloaded state shown in FIG. 1, as shown in FIG. 6B, the rib (block) 103 of the shoulder 104 of the tread 101 contacts a road surface 200. At this time, the buttress 120 deforms such that the thin groove 121 is closed. A tire transverse direction outermost portion P, which is at a shoulder side curved portion 131 of a tire contour portion 130 formed between the end portion 105 of the shoulder 104 and a groove floor 121a of the thin groove 121, is in a state of projecting outwardly, i.e., a state in which it is not covered by the buttress 120.
From the state shown in FIG. 6B, when the truck or the like, to which the pneumatic radial tires 110 are mounted, turns at an extremely low speed, lateral force Y arises. Due to this lateral force Y, the pneumatic radial tire 110 deforms such that the buttress 120 contacts the road surface 200. Accordingly, the tire transverse direction outermost portion P of the shoulder side curved portion 131 of the tire contour portion 130 contacts the road surface 200, and then catches on the road surface 200 due to the lateral force Y. Because the thin groove 121 is closed by the buttress 120, the rib 103 of the shoulder 104 and the buttress 120 form rigid bodies. A difference in rigidities arises at a portion of the thin groove 121 closed by the buttress 120, i.e., a portion where a sidewall side end portion Q of a sidewall side groove wall surface 121b of the thin groove 121 contacts the tire contour portion 130, and a region further toward the road surface 200 side of the sidewall side end portion. Namely, cracks are formed from a portion near a border of the difference in rigidities, and there is the problem that the rib 103 of the shoulder 104 tears off.